This invention relates to gas furnaces and, in particular, to gas furnaces employing a plurality of main burners and a carryover tube for providing a flame for igniting these burners.
In one type of gas furnace employing a carryover tube, a spark igniter is disposed at a first end of the carryover tube and a flame sensor is disposed at a second end of the tube. Upon a call for heat from the thermostat of the furnace, the furnace ignition control causes a combustible gas to be coupled to the carryover tube. At this time, the control also causes actuation of the spark igniter, which then ignites the gas resulting in a flame at the first end of the tube. This flame then propagates to the second end of the carryover tube and is sensed by the flame sensor. The flame sensor reports this condition to the ignition control and, in response, the control causes combustible gas to be simultaneously coupled to the main burners. The flame from the carryover tube then ignites this gas and the burners are thereby simultaneously ignited to produce the heating flame for the furnace.
Because the above type furnaces employ spark igniters, they are inherently susceptible to spark gap position. Furthermore, the use of a spark results in a comparably high density energy source in a small area, which can be promotive of an explosion.
Another type of furnace utilizing a carryover tube, employs a hot surface igniter such as, for example, a silicon carbide igniter. The use of a hot surface igniter is advantageous, since this type of igniter has a low susceptibility to misalignment, a large surface area and a high surface energy which promotes smooth lighting. In this type of furnace, the carryover tube and the main burners are both initially supplied gas simultaneously. As a result, igniting of the carryover tube by the igniter causes direct ignition, i.e., substantially simultaneous ignition of the carryover tube and burners. Because of this, the furnace is required to also perform a delayed ignition test which is designed to satisfy ANSI standards Z21.47 and Z21.64.
In order to carry out such test, it is typical for the furnace to utilize a so called "step" gas valve for supplying gas to the burners and the carryover tube. This valve allows for simultaneous ignition of the carryover tube and main burners at an initially reduced pressure (e.g., 50 percent of normal operating pressure) and then the valve steps to the normal operating pressure in a short period of time, typically six to ten seconds. As a result of the reduced pressure at initial ignition, the potential for concussion is reduced in the event of delayed ignition.
The use of "step" gas valves, however, is disadvantageous, since they are costly, large in size and have a relatively great number of parts. Furthermore, they are less desirable in the event a furnace must be converted from natural gas to liquid propane. In such case, step valves, due to their construction, are not easily modified and usually the entire valve must be changed. This drives up the cost of the conversion kit as compared to furnaces which employ standard snap open or slow open valves which are more easily converted.
It is, therefore, an object of the present invention to provide a gas furnace which overcomes the above-mentioned disadvantages.
It is a further object of the present invention to provide a gas furnace which has less potential for delayed ignition.
It is yet a further object of the present invention to provide a gas furnace which can be converted from natural gas to liquid propane in a cost effective manner.
It is a further object of the present invention to provide a gas furnace which is adapted to utilize more cost effective gas valves.